GNSS Interference

The GPS signal is a low power signal. It is comparable
to the power emitted by a 60W light-bulb located more than 20,000 km away from the
surface of the earth. This means that the signal could easily be disturbed by
any ground source located near an aircraft and emitting in the GPS L1 frequency
band (1575.42 MHz +/-10 MHz), leading to the loss of GPS data (fig.1).

Personal
Privacy Devices (PPD)

Some of the reported disturbances were caused by
portable Personal Privacy Devices that jam a GPS signal in the immediate area to
avoid tracking. Operational disruptions at airports due to a loss of the GPS
signal in the area around the airports have been caused when these devices were
activated in the vicinity of an airport.

Protection
of sensitive sites and VIPs

Certain sensitive
sites may be protected using GNSS RFI for security reasons, such as correctional
facilities or sites where dignitaries or political figures are living or
visiting. Aircraft operating in the vicinity of these sites may be affected by
interference with the GPS signal.

GPS repeater

GPS repeaters
are used to make a GPS signal available inside a hangar during aircraft
maintenance. GPS repeater signals have caused interference with actual GPS
signal in some reported events, causing reception issues on aircraft located
close to the hangar.

TV
broadcast station malfunction

A TV broadcast
station malfunction reportedly disturbed the GPS signal and affected aircraft
operations.

Military
GPS RFI in conflict zones

GPS RFI can also cause loss of the GPS signal in flight
if too close to areas of military conflict. These areas are often known and
NOTAMs inform flight crews that they may encounter interference close to these areas.
It can be the case that military RFI activity is not known in advance or
communicated leading to loss of GPS signal without prior notice.

GNSS RFI can cause the loss of GNSS position and
timing. Even if GNSS plays a major role in the aircraft positioning system,
Airbus aircraft are designed to be robust to GNSS signal loss. The use of other
sources of data (IRS, VOR and DME) enables the aircraft systems to maintain a
position computation capability. A loss of GNSS inputs does not lead to a map
shift or to an erroneous position computation by the Flight Management Systems (FMS).
In the case of a loss of GPS signal, the FMS switches from the mixed GPS/IRS
position to an IRS-DME/DME position or IRS-VOR/DME or pure IRS in order of
priority. Refer to the FCOM description of the FMS position computation for
more detail. 

Certain navigation and surveillance functions or operational
capabilities may be lost if there is a loss of the GPS signal (fig.2). This is because the need for high
accuracy and integrity of GPS position data is not met (e.g. for RNP AR, SLS,
GLS, etc.) or when functions rely only on available GPS data for position or
time reference. All, or some of the cockpit effects listed in (fig.2) may be triggered in an order
that depends on the confirmation time of each system’s monitoring function and
how long the GPS signal is lost. 

GPS signal loss and its associated effects were temporary
in most of the reported events. The lost functions and capabilities were
recovered immediately after aircraft moved out of range from the source of the
radio-frequency interference.  

Check
RFI NOTAMs

Operators should consider the NOTAM related to known or
expected areas with GNSS RFI when planning flights. If a NOTAM is applicable to
the flight, then the availability of non GNSS-Based routes, procedures and approaches
(such as ILS, VOR and DME) must be checked for the affected area.

Application
of ECAM/FCOM procedures

Flight
crews must follow the associated ECAM or FCOM procedures if a loss of
GNSS signal occurs during flight with a cockpit effect described in (fig.2).

Recovery of the signal    

Loss of GPS information is usually temporary and the
normal navigation mode based on GPS data (“GPS PRIMARY” or “NAV PRIMARY”), as
well as the communication and surveillance functions, are recovered as soon as
the aircraft leaves the area affected by RFI. It is therefore not necessary to deselect
GPS in the case of RFI as this would prevent the aircraft from recovering its
full capabilities when the GPS signal is restored.

Zones with ADS-B OUT required

If the GNSS loss occurs in an area where ADS-B OUT is
required per regulation, the flight crew should notify ATC of the loss of ADS-B
OUT and report that this is due the loss of the GNSS signal.

Report
to Maintenance

At the end of a flight where the effects of a loss of
GNSS signal were experienced, the flight crew should report the event and
cockpit effects to the Maintenance to investigate and confirm if the event was
due to RFI or a result of a system or equipment malfunction.

Share
information

Operators should report any identified suspected GNSS
RFI events to regional (e.g. ANSPs) and international organizations, such as EUROCONTROL’s
Voluntary ATM Incident Reporting (EVAIR). This will facilitate and accelerate
GNSS RFI event confirmation or resolution, and enable the publication of a
NOTAM to share information to all other operators flying near the affected
area.

At the end of a flight impacted by a transient loss of
GNSS, a confirmation should be done to make sure that the effects encountered
were due to RFI and not to a system or equipment malfunction.

At the end of a flight affected by transient GNSS loss within
an area with known RFI, Airbus recommends that maintenance personnel reset the system
and test both Multi-Mode Receivers (MMR).

To ensure that there was no system failures, Airbus
also recommends a system test of any equipment affected by a loss of GNSS
signal based on the cockpit effects observed during the flight. Should any
system test fail, maintenance personnel must perform troubleshooting in
accordance with the associated Trouble-Shooting Manual (TSM) task.

Refer to the “GNSS loss and
GNSS interference on Airbus aircraft” ISI article ref 34.36.00049 available on
the Airbus World portal for more details and a list of the related AMM/MP tasks
for system tests.

What if the
interference is still present on ground?

It the GNSS is still impacted by RFI on ground, the
aircraft should be moved out of the RFI area. A dispatch under MEL conditions should
be considered if this is not possible to do so.

At the end of a flight affected with transient GNSS
loss within an area without known RFI issues, Airbus recommends that maintenance
personnel confirm the root cause of the GNSS loss by studying all potential
sources: aircraft system failure, GNSS constellation anomaly, environment
masking, multipath or space weather events such as ionospheric scintillation.
When all these potential causes are eliminated, RFI can be suspected. In this
case, aircraft data should be sent to Airbus for further investigation. A list
of the information items to report is provided in the “GNSS loss and GNSS interference on Airbus aircraft” ISI article ref
34.36.00049, available on the Airbus World portal.